What can I get done to achieve a "crease" under my butt cheeks?

Hello dear!!
Thanks for the question and provided information as well.
Unfortunately, without pictures, it is very difficult to give you a proper answer.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

This is something that talented plastic surgeons are able to achieve with this procedure. Consult with a board-certified plastic surgeon to determine whether this is a plausible outcome for you and whether your PS is experienced in this specific work.